UPDATE: Fortune reports today that the lead researcher on this study, Jerry Bromenshenk, had financial ties to Bayer Crop Science—including a research grant—that were not disclosed. Bayer makes pesticides that some beekeepers and researchers have cited as a possible cause of colony collapse disorder, and Bromenshenk’s conclusions in this study could benefit the company. Bromenshenk says the money did not go to this project or influence its findings.

Viruses. Mites. Fungi. Genetically modified crops. Inbreeding because of industrial agriculture. They’ve all been floated as possible causes of colony collapse disorder (CCD), the mystery affliction that’s been wiping out honeybees, and by doing so threatening the agricultural industries that rely on those insects. Despite the flood of reports since 2006 about these suspects (and more absurd ones, like cellphone radiation disorienting the bees), the bee die-off continues without a clear explanation.

A study out this week in PLoS One points the finger in a new direction. What’s interesting about this explanation is its contention that there’s tandem foul play at work in CCD—two of the suggested culprits could be working together. But the mystery isn’t solved just yet.

The scientists from the U.S. Army and the University of Montana implicated the dual threat of an invertebrate iridescent virus, or IIV, and fungi called Nosema. Previous studies had found both associated with CCD independently. When these researchers looked at bees from collapsed colonies, healthy colonies, and healthy colonies that collapsed during the course of the study, they found that it was both the virus and the fungi together—but not simply one or the other—that was connected to a colony collapse. From their study:

Interestingly, the presence or absence of IIV in a given honey bee colony may explain why in the USA N. ceranae sometimes seems to contribute to severe colony losses (IIV present), and sometimes not (IIV absent), as reported both by researchers and beekeepers.

But which one is the key?

“It’s chicken and egg in a sense — we don’t know which came first,” Dr. [Jerry] Bromenshenk said of the virus-fungus combo — nor is it clear, he added, whether one malady weakens the bees enough to be finished off by the second, or whether they somehow compound the other’s destructive power. “They’re co-factors, that’s all we can say at the moment,” he said. “They’re both present in all these collapsed colonies.” [The New York Times]

The scientists are tip-toeing carefully in their statements, because despite the headlines that come out whenever a new CCD study emerges (see the Times’ “Scientists and Soldiers Solve a Bee Mystery“), bee researchers are still wading through a mess of correlation versus causation:

“We truly don’t know if these two pathogens cause CCD or whether the colonies with CCD are more likely to succumb to these two pathogens,” Jerry J. Bromenshenk of the University of Montana said in a statement. [AP]

Also, there may very well be more than two factors at play, especially considering the pile of other things scientists have connected to colony collapse disorder during the last four years. Previous studies that implicated viral infections in bees suggested that perhaps the infections prevented them from producing proteins that would help them resist parasites or pesticides. Those researchers are working to figure out the functions of the genes affected.

In the case of Bromenshenk’s team, the scientists’ follow-up task to this week’s study is to isolate the IIV they found and try to use it in inoculation experiments, hoping that could reveal whether the virus is a key player in causing CCD or just an invader after the fact.

I just saw a new documentary about bees. Queen of the Sun is about beekeeping, industrial agriculture, and Colony Collapse Disorder. I highly recommend this film. It’s by the director of The Real Dirt on Farmer John.